1. Field of the invention
The present invention relates to a method of and a device for demagnetizing a magnetic recording medium, and more particularly to a method of and a device for removing residual magnetism from a magnetic recording medium with a plurality of erasing heads.
2. Description of the Prior Art
Signals are generally recorded on magnetic recording mediums such as magnetic tapes, magnetic disks, or the like by residual magnetism. The recorded signals can be erased by removing the residual magnetism under a magnetic field generated by an erasing head positioned closely to the magnetic recording medium.
One of the conventional ways for erasing the recorded magnetic signals is known as a DC erase process which utilizes either an erasing head with a direct current flowing through its coil or an erasing head having a permanent magnet. Such an erasing head produces a magnetic field in a prescribed direction to magnetize the magnetic recording medium uniformly until it is magnetically saturated. The DC erasing head is however disadvantageous in that it leaves noises of high level on the magnetic recording tape when erasing the recorded signals therefrom, thus increasing the distortion rate of signal waveforms which will be recorded and reproduced.
Another erasing process has been to use an AC erasing head having a coil supplied with an alternating current for magnetizing the magnetic recording medium as it passes the erasing head until the magnetic recording medium, is saturated. As the magnetic recording medium travels away from the AC erasing head, the recording medium is less subject to the alternating magnetic field produced by the AC erasing head, and hence the residual magnetism on the recording medium is progressively reduced and finally removed.
Where the magnetic recording head has a high coercive force, the AC erasing head is required to be supplied with an increased amount of electric power. When the AC erasing head is supplied with such increased electric power, however, it can produce only an insufficient magnetic field due to an eddy-current loss of the core, and cannot sufficiently eliminate the residual magnetism from the magnetic recording medium.